2V90 Biaxial Ellipsoid

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In the case of minerals that have ‘2V90’, the two circular sections are mutually perpendicular, and hence the two optic axes are also exactly 90° apart - this is the ‘2V angle’. The shape of the ellipsoid can neither be described as prolate nor oblate and is on the cusp in between. More detail on the derivation of the biaxial indicatrix can be found here. Compare this with the optically positive biaxial indicatrix and the optically negative biaxial indictrix.